The present invention relates to a process for the introduction of water-insoluble photographic additives, especially dye couplers, into aqueous media, under application of high boiling organic solvents.
In manufacturing of photographic materials, numerous hydrophobic components, especially dye couplers, stabilizers, optical brighteners etc. must be introduced into hydrophilic materials. As known, this is accomplished by dissolving the hydrophobic substances in an organic, high boiling solvent and dispersing this solution in an aqueous medium. Low-boiling auxiliary solvents, such as ethyl acetate or methylene chloride are often added, which are removed after dispersion. A dispersion is obtained wherein the dye couplers are present in the form of extremely fine droplets in the hydrophilic bonding agents of the photographic layer. Gelatin is preferably used as a hydrophilic bonding agent but other polymeric bonding agents may also be employed.
The high boiling solvents which are used for the dispersion of dye couplers must fulfill a series of requirements, including possession of the necessary good dissolving capacity for the couplers and prevention of crystallizing out of the dye couplers and of the image dyestuffs obtained after photographic development. They need to be compatible with the photographic development baths and they should not present any signs of separation from the bonding agents, even after a long period of storage, which would lead to clouding and, thereby, to a lowering of quality. Numerous high-boiling solvents for the dispersion of photographic dye couplers have been known. Thus DE-OS No. 2,129,684 teaches the use of Formamide, U.S. Pat. No. 2,533,514 the use of Dibutyllaurylamide and DE-OS No. 2,629,842 the use of benzoic acid ester.
It has also been known to utilize phthalic acid esters, tricresylphosphate or, according to DE-OS No. 2,042,581, phosphoric acid esters which in addition to aryl groups, also contain aliphatic groups, such as, for example, di-(n-hexyl)-phenylphosphate or tris-(2-ethyl-hexyl)-phosphate.
The solvents that are usually employed in the introduction process have a series of disadvantages. Thus they have insufficient dissolving capacity for the dye couplers and often need to be added in high concentrations, which leads to low dispersion stability and thereby to difficulties, especially in storage.
Further, it has been proposed (WP G 03 C/218404) to utilize phosphoric acid triamides or mixed phosphoric acid ester amides as high-boiling solvents for the introduction of hydrophobic couplers. Stable dispersions result. Utilization of the mentioned phosphoric acid amides is, however, restricted to certain types of dye couplers so that it is desirable to find new high-boiling solvents that have a wide spectrum of different hydrophobic dye couplers with which dispersions suitable for storage can be manufactured.
The object of the invention is to develop a process for the introduction of hydrophobic photographic additives, especially dye couplers, into gelatin or other hydrophilic bonding agents, which averts the disadvantages of the known processes, for instance, too low a dispersibility and, thereby, insufficient stability on storage.
The invention has the object of creating stable, permanent dispersions of photographic dye couplers through the utilization of a new high-boiling organic solvent.